This invention relates to automatic tracing systems for controlling the operation of machine tools. In particular, this invention relates to a velocity control device capable of achieving both high accuracy and high speed of operation.
In conventional tracing systems, the deflection of a tracing stylus relative to a template produces control signals which are used to guide the movement of the stylus around a contour of the template and, correspondingly, to position a machine tool relative to a workpiece such that the tool machines an identical contour around the workpiece. Transducers in the stylus head produce signals corresponding to the stylus deflection relative to a reference coordinate system, whereby the magnitude and direction of the deflection of the stylus as it traces the contour of the template are resolved into its vector components. As the deflection vector is ideally normal to the template contour, and the movement of the stylus is always tangential relative to the template, the orientation of the velocity command vector for the tracing movement is derived by rotating the deflection vector 90.degree..
The magnitude of the velocity command vector is generally set to a desired tracing speed for the system. In conventionally tracing systems the set speed is typically a compromise value balancing the advantages of high-speed tracing against the need for accurate machine response. If the speed is too high, the system cannot accurately respond to sharp changes in the contour of the template. For example, when negotiating an inside corner of the template at high speed, the abrupt change of direction may result in an excessive deflection error before the system can recover, thereby causing the tool to overcut the workpiece. Thus, the full tracing speed capacity of the system is not entirely realized because of the need to accommodate sharp corners in the template contour. Operation at a lower tracing speed means that the system will be slower and less productive. Attempts to provide an increased response capability have resulted in rather complex systems, for example, using an eccentric stylus with direction-of-motion controls or tracing systems offset from the operation of the machine tool.